2U2 JiEl'OKT — 18U-J. 



through eitliei' to the inducing current in the other is called the mutual 

 inductance of the circuits. 



(20) Of two coils, with ii and ?t' turns respectively, the total mutual 

 inductance is to be reckoned for every turn of wire on each coil, and is 

 therefore nn times the inductance of the mean turn of one coil on the 

 inean turn of the other. 



(21) The mutual inductance of two coils is i:nn7i' times the permeance 

 of the largest magnetic solenoid which threads both. For if every turn 

 of one conveys a current C, while every turn of the other surrounds an 

 induction N' in consequence, the permeance of the magnetic solenoid 

 threading the second coil is P = N'/-i7i'?iC ; but the total effective mutual 

 induction, MO, through all the turns is w'N' ; hence M=47r7i?i'P. 



(22) When two coils each conveying one ampere are constantly con- 

 nected by one henry of mutual inductance, the kinetic energy of the field 

 due to their mutual action is one joule. 



(23) If the st'//'-induction coefficient of a coil is being considered, its 

 total inductance may be taken as 7i- times the inductance of the mean 

 turn ; that is n times the total induction through it divided by the in- 

 ducing current. Or the weber-turns per ampere give the self-inductance 

 in henrys. 



(23a) The expression weber-turns, to signify the i^roduct of the total 

 field into the number of spires surrounding it, though at first sight not in 

 precise correspondence with the phrase ampfere-turns where the current 

 cii'culates in the spiral instead of forming its core, is really accordant 

 with it, because a spiral and its core are geometrically interchangeable. 



(24) The practical unit of inductance, whether self or mutual, is called 

 a henry ; and a coil of n turns has a henry of inductance, on itself or on 

 another of w' turns, when an ampere in one maintains 1/w' weber of 

 induction (through itself or) through the other. 



(25) When the induction through a coil varies for any reason at the 

 rate of one weber per second, the E.M.F generated in each turn is one volt. 



(26) When the inductance of a soil is one henry, on itself or on another, 

 a small variation of current in it at the rate of one ampere per second 

 induces an EMF of one volt in itself or in the other. 



(27) When the inductance of a coil conveying one ampere varies at 

 the rate of a heiiry j^er second, the induced E.M.F is one volt. 



(28) When the self-inductance of a coil is constantly, or on the 

 average, one henry, while an ampere current is genei'ated in it, the 

 kinetic energy of the field due to that ampere is half a joule. 



Second si/stem. 



(29) If instead of taking a gauss as equal to a c.g.s. unit of magnetic 

 potential, we take the circulation of H caused by one ampere-turn as 

 the practical unit of magneto-motive force, we shall have 1 ampere-turn 



= — cg.s. units of gaussage. 



(30) The practical unit of permeance will then be that in which a 

 weber of total induction is excited by each amp)ere-turn ; in other words, 

 it will be Att x 10^ c.g.s. units of permeance. 



(31) And the practical unit of inductance will be that of a coil in 



"which an ampere in every turn excites -th of a weber through every 



11 





